Nano alumina compositions are generally well-known in the art. In particular, U.S. Pat. No. 2,915,475 describes a process for producing nano alumina fibrils by heating alumina in an aqueous acid dispersion to produce fibrous alumina monohydrate having a boehmite crystal lattice. As discussed in detail in the aforementioned U.S. patent, many factors affect the formation and size of the alumina fibrils, including process time and temperature, acid concentration, and alumina concentration. Additionally, U.S. Pat. No. 2,915,475 discloses that the nano alumina fibrils produced by the process discussed therein can be used for a variety of applications, including as a thickening, emulsifying, dispersing, and suspending agent, as well as a filler for elastomeric materials and plastics for improving strength and/or abrasion resistance, including tensile and impact strength. However, one of the major drawbacks of using nano alumina fibrils, such as those described in U.S. Pat. No. 2,915,475 as additives in polymeric materials, in particular in polyolefinic materials, is that the fibrils agglomerate within the polyolefinic material. This results in a reduced dispersion of the nano alumina fibrils, as well as a reduced interfacial adhesion between the polyolefinic material and the fibrils, which results in the polyolefinic material having reduced physical and mechanical properties, especially with respect to polyethylene films. Additionally, nano alumina fibrils can negatively affect the optical properties of polyolefinic materials, such as polyethylene films, including reducing transparency, increasing haze, and increasing undesirable coloring of the material.
In order to overcome these deficiencies, International Application Publication WO 2006/131450 relates to polyolefin nanocomposite materials having a polyolefin and at least one nanosize mineral filler. However, not only do the polyolefin nanocomposite materials disclosed therein generally relate to propylene polymer compositions, but a myriad of compounds are described as being suitable nanosize fillers, including nanohydrotalcites and phyllosilicates, with smecitite clays, kaolin clay, attapulgite clay, bentonite clay, and montomorillonite clays being preferred.
Additionally, International Application Publication WO 2008/097086 relates to stretched polyolefin materials having nano-nucleating agents therein. However, as with WO 2006/131450, not only does International Application. Publication WO 2008/097086 preferably relate to polypropylene materials, but the nano-nucleating agents are generally described as being selected from a variety of compounds, including natural or synthetic nanoclays, modified nanoclays, zeolites, alumina, silica, and fibrous or needle-shaped materials, including metal whiskers, carbon whiskers, or nanotubes. In particular, suitable nano-nucleating agents are described as smectite clays, including montmorillonite, and needle-shaped materials, as well as zeolites, including ZSM-5, zeolite beta, mordenite, ferrierite, and zeolite Y.
Furthermore, German Patent Application DE 10 2004 009 582 relates to boehmitic aluminum hydroxide nanocrystals as fillers for polymeric material. However, as with WO 2006/131450 and WO 2008/097086, German Patent Application DE 10 2004 009 582 relates to various boehmitic aluminum hydroxide nanocrystals, including fibrous particle shapes, as well as a wide variety of polymeric materials.
Contrastingly, work done by Rolf Mülhaupt, Boehmite Nanorod-Reinforced-Polyethylenes and Ethylene/1-Octene Theremoplastic Elastomer Nanocomposites Prepared by In Situ Olefin Polymerization and Melt Compounding, Journal of Polymer Science: Part A: Polymer Chemistry. Vol. 46. 2755-2765, 2008, relates to nanocomposites of high-density polyethylene (HDPE) and poly(ethylene-co-1-octene) thermoplastic elastomers having nano boehmite fillers therein. However, not only do the nano boehmite fillers need to be incorporated in situ in the olefin polymerization process, but various nano boehmites are discussed, with the preferred nano boehmite fillers being rod-shaped. Additionally, for improved strength, the resultant polyethylene materials have at least 4% by weight, all the way up to 75% by weight, of the nano boehmite fillers present.
Therefore, there remains a need in the art for polyethylene compositions and films having improved strength and optical properties, with a minimum amount of nano boehmite nucleator present.